I. Field of the Invention
The present invention relates to wireless telecommunications systems. More particularly, the present invention relates to a novel and improved method and apparatus for mitigating the effects of destructive interference between the respective synchronization channels broadcast by two or more base stations in a code-division multiple access system.
II. Description of the Related Art
In a wireless radiotelephone communication system, many users communicate over a wireless channel. Communication over the wireless channel can be one of a variety of multiple access techniques that allow a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeatersxe2x80x9d, issued Feb. 13, 1990, assigned to the assignee of the present invention, and incorporated herein by reference. An exemplary CDMA system is further described in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSystem And Method For Generating Signal Waveforms In A CDMA Cellular Telephone Systemxe2x80x9d, issued Apr. 7, 1992, assigned to the assignee of the present invention, and incorporated herein by reference.
Recently, third-generation (3G) CDMA communication systems have been proposed including proposals such as cdma2000 and W-CDMA. These 3G CDMA communication systems are conceptually similar to each other with some significant differences. One significant difference is that in the cdma2000 system, each of the base stations operates synchronously. In other words, each base station in the cdma2000 system operates according to the same universal time reference. Each base station transmits a pilot channel having the same PN spreading code, but having a different PN phase offset. As a result, a mobile station can acquire the pilot channel of one or more base stations by searching through the possible PN phase offsets of the known PN spreading code. Additionally, the mobile station can distinguish among different base stations by their respective PN phase offsets, even though they are using the same PN spreading code.
However, under the currently proposed W-CDMA system standard, each of the base stations operates asynchronously. In other words, there is no universal time reference among separate base stations. In the W-CDMA system, each base station transmits a xe2x80x9csynchronizationxe2x80x9d channel that comprises two sub-channels. The first of the two sub-channels, the primary synchronization channel, uses a primary synchronization code, cp, that is common to all base stations. The second of the two sub-channels, the secondary synchronization channel, uses a cyclic set of secondary synchronization codes, cs, that are not shared by other base stations that are not in the same code group. The mobile station in a W-CDMA system can acquire the synchronization channel of one or more base stations by searching for the primary synchronization code, cp of the primary synchronization channel, and then using the timing information derived from the primary synchronization channel to process the secondary synchronization channel.
FIG. 1 is a timing diagram illustrating the structure of the synchronization channel (SCH) of a W-CDMA system. In FIG. 1, one frame is illustrated. The one frame comprises sixteen individual slots, separated in FIG. 1 by dashed lines. The primary synchronization channel is shown as a burst 100 of the primary synchronization code, transmitted at the beginning of each slot. The secondary synchronization channel is shown as a burst 102 of one of 17 possible secondary synchronization codes, transmitted in parallel with the primary synchronization code at the beginning of each slot.
The primary synchronization channel comprises an unmodulated code that is the same for every base station in the system, and is transmitted time-aligned with the slot boundary of the transmitting base station. The secondary synchronization channel comprises a sequence of 16 unmodulated code words that are orthogonal to each other and to the primary synchronization code. Each secondary synchronization code word is chosen from a set of 17 different orthogonal codes. The sequence on the secondary SCH indicates which of the 32 different code groups the base station PN scrambling code belongs to. 32 sequences are used to encode the 32 different code groups each containing 16 scrambling codes. The 32 sequences are constructed such that their cyclic shifts are unique. In other words, a non-zero cyclic shift less than 16 of any of the 32 sequences is not equivalent to some cyclic shift of any other of the 32 sequences. This property is used to uniquely determine both the long code group of the base station and the frame timing. It should be noted that the term xe2x80x9cscramblingxe2x80x9d code as used with reference to a W-CDMA system is synonymous with the term xe2x80x9cspreadingxe2x80x9d code as used above with reference to a cdma2000 system. However, for consistency and clarity of disclosure with respect to W-CDMA based systems, the terminology xe2x80x9cscramblingxe2x80x9d code will be used herein to denote the code used to spread the information signal over the desired bandwidth.
During cell search, the mobile station searches for the base station to which it has the lowest path loss. It then determines the downlink scrambling code and frame synchronization of that base station. The cell search begins by using the synchronization channel. During the first step of the cell search procedure, the mobile station uses the primary SCH to acquire slot synchronization to the strongest base station. This may be done with a single matched filter matched to the primary synchronization code, cp, which is common to all base stations. During the second step of the cell search, the mobile station uses the secondary SCH to find frame synchronization and identify the code group of the base station found in the first step. This is done by correlating the received signal with all possible (16) secondary synchronization codes. Specifically, the mobile station correlates the sequence of 16 code words that are received against the 32 possible sequence patterns and 16 possible cyclic shifts, for a total of 32xc3x9716 possibilities. During the third and last step of the initial cell search, the mobile station determines the exact PN scrambling code used by the found base station. The scrambling code is identified through symbol-by-symbol correlation of the pilot symbols received over one or more common channels with the PN scrambling codes that belong to the code group identified by the second step.
A functional block diagram of the multiplexing of the synchronization channel (SCH) with the other downlink physical channels (dedicated channels) is shown in FIG. 2. In FIG. 2, ones generator 202 generates a sequence of logical one values for 256 bits at the beginning of each slot. To be more precise, ones generator 202 generates the complex signal 1+j1. These ones are complex spread in complex spreader 208 with the primary synchronization code, cp, from primary code generator 206. The primary synchronization code is common to all base stations. Together the ones generator 202, primary code generator 206 and complex spreader 208 may be referred to as a xe2x80x9cprimary synchronization channel generatorxe2x80x9d.
Ones generator 204 (which may be the same as ones generator 202) also generates a sequence of logical one values for 256 chips at the beginning of each slot. The ones are complex spread in complex spreader 210 with the secondary synchronization code, cs, from secondary code generator 212. Together, the ones generator 204, complex spreader 210, and secondary code generator 212 may be referred to as a xe2x80x9csecondary synchronization channel generatorxe2x80x9d. The in-phase (I) and quadrature-phase (Q) components of the primary SCH and the secondary SCH are then respectively combined in combiner 214 to form the synchronization channel (SCH). Dedicated channel data is complex spread in complex spreader 218 with a scrambling code, cscramb, which is also unique to the particular base station. The scrambled dedicated channel data is combined with the SCH in combiner 216 and forwarded to an I/Q modulator (not shown) for modulation.
As can be seen from FIGS. 1 and 2, the presently proposed SCH of the W-CDMA system is transmitted with a zero phase offset. Since the base stations in a W-CDMA system operate asynchronously, there will be regions within the coverage area of multiple base stations where the primary SCH from multiple base stations will arrive at the mobile station with the same time alignment. When this happens, detection of the primary SCH timing could become difficult for the mobile station. In the worst case, the primary SCH""s from different base stations would arrive at the mobile station so that they destructively interfere with one another, preventing the mobile station from acquiring primary SCH. Furthermore, if the propagation environment is changing slowly, such a state of destructive interference could persist for a considerable length of time. This is of particular concern when the mobile station is stationery, such as in a wireless local loop (WLL) system, or when the mobile station is otherwise moving relatively slowly.
What is needed is a method and apparatus for mitigating this prolonged state of destructive interference caused by code timing collisions.
The present invention is a novel and improved method for mitigating the effect of interference between a first base station and a second base station, the first base station and second base stations both sharing a same primary synchronization code. The method includes generating a primary synchronization channel having the primary synchronization code. In a W-CDMA system, all base stations share this primary synchronization code. It is this sharing of a common primary synchronization code that causes code timing collisions. To mitigate the effects of these collisions, the method of the present invention includes rotating the primary synchronization channel in phase according to a phase rotation sequence before transmitting the primary synchronization channel. By rotating the primary synchronization channel in phase according to the phase rotation sequence, instances of prolonged destructive interference may be reduced.
In the preferred embodiment the phase rotation sequence is pseudorandom in phase, but for simplicity, includes changing phase by integer multiples of xcfx80/2 radians. However, it may include pseudorandomly changing phase by any arbitrary angle. With regard to timing, the phase rotation sequence may include changing phase once per slot, or alternately once per frame. However, it may include changing the phase at any arbitrary periodicity so long as it is changed at a slot boundary (i.e., not during the middle of a slot).
In the exemplary W-CDMA system, method also includes generating a secondary synchronization channel having a secondary synchronization code, with the phase rotation sequence being based at least in part on the secondary synchronization code. Basing the phase rotation sequence at least in part on the secondary synchronization code allows for convenience since the secondary synchronization code is already present for other purposes. Additionally, in areas where base stations in different code groups are adjacent to each other, basing the phase rotation sequence at least in part on the secondary synchronization code (which is not shared among base stations in different code groups) will minimize the duration of interference.
In various embodiments, the method includes combining the primary synchronization channel and the secondary synchronization channel to produce a synchronization channel. In a first embodiment, the step of rotating the primary synchronization channel in phase comprises rotating the primary synchronization channel before the combining step. Thus, the first embodiment rotates only the primary synchronization channel and not the secondary synchronization channel. In a second embodiment, the step of rotating the primary synchronization channel in phase comprises rotating the synchronization channel in phase. Thus, the second embodiment rotates both the primary and secondary synchronization channels after they have been combined. In yet another embodiment, the synchronization channel is combined with a dedicated channel to produce a downlink channel that is then rotated in phase.
The present invention also includes an apparatus for performing the method summarized above. The apparatus includes a primary synchronization channel generator for generating a primary synchronization channel having the primary synchronization code; a phase rotator, coupled to the primary synchronization channel generator, for rotating the primary synchronization channel in phase according to a phase rotation sequence; and a transmitter, coupled to the phase rotator, for transmitting the primary synchronization channel.
In a first embodiment, the apparatus further comprises a first combiner for combining the primary synchronization channel and the secondary synchronization channel to produce a synchronization channel; wherein the phase rotator is coupled between an output of the primary synchronization channel generator and an input of the first combiner. In a second embodiment, the phase rotator is coupled to an output of the first combiner. In a third embodiment, a second combiner combines the synchronization channel and a dedicated channel to produce a downlink channel, and the phase rotator is coupled to an output of the second combiner.